Remove redundant cast

Remove redundant cast needed from when net5.0 was a TFM.

Author: martincostello

src/HttpClientInterception/HttpRequestInterceptionBuilderExtensions.cs

@@ -181,9 +181,7 @@ public static HttpRequestInterceptionBuilder WithFormContent(
 
         async Task<byte[]> ContentFactoryAsync()
         {
-            // The cast is to make nullability match for .NET 5.0.
-            // See https://github.com/dotnet/runtime/issues/38494.
-            using var content = new FormUrlEncodedContent((IEnumerable<KeyValuePair<string?, string?>>)parameters);
+            using var content = new FormUrlEncodedContent(parameters);
             return await content.ReadAsByteArrayAsync().ConfigureAwait(false);
         }